The invention relates to dispersing gas into molten metal and, more particularly, to techniques for causing finely divided gas bubbles to be dispersed uniformly throughout the molten metal.
In the course of processing molten metals, it sometimes is necessary to treat the metals with gas. For example, it is customary to introduce process gases such as nitrogen and argon into molten aluminum and molten aluminum alloys in order to remove undesirable constituents such as hydrogen gas, non-metallic inclusions, and alkali metals. The process gases added to the molten metal chemically react with the undesired constituents to convert them to a form (such as a precipitate or a dross) that can be separated readily from the remainder of the molten metal. In order to obtain the best possible results, it is necessary that the process gas be combined with the undesirable constituents efficiently. Such a result requires that the gas be dispersed in bubbles as small as possible and that the bubbles be distributed uniformly throughout the molten metal. When removal of hydrogen gas is desired, the process gas bubbles allow hydrogen atoms to diffuse into the bubble and form a hydrogen molecule. Then the bubbles rise to the surface where the hydrogen can be released to the atmosphere or to the dross phase or flux cover.
As used herein, reference to “molten metal” will be understood to mean any metal such as aluminum, copper, iron, and alloys thereof, which are amenable to gas purification. Further, the term “gas” will be understood to mean any gas or combination of gases, including argon, nitrogen, chlorine, freon, and the like, that have a purifying effect upon molten metals with which they are mixed.
Heretofore, gases have been mixed with molten metals by injection through stationary members such as lances, or through porous diffusers. Such techniques suffer from the drawback that inadequate dispersion of the gas throughout the molten metal can occur. In order to improve the dispersion of the gas throughout the molten metal, rotating injectors are commonly used, which provide shearing action of the gas bubbles and intimate stirring/mixing of the process gas with the liquid metal.
Despite the existence of combined rotating/injecting devices, certain problems remain. Combined devices often exhibit poor mixing action. Sometimes cavitation occurs or a vortex is established that moves around the inside of the vessel within which the molten metal is contained. Frequently these devices dispense bubbles that are too large or which are not uniformly distributed throughout the molten metal. A problem with one known prior device is that it utilizes an impeller having passageways that can be clogged with dross or foreign objects. Most of the prior devices are expensive, complex, and usable with only one type of molten metal refining system. Other problems frequently encountered are poor longevity of the devices due to oxidation, erosion, or lack of mechanical strength. These latter concerns are particularly troublesome in the case of aluminum because the rotating/injecting devices usually are made of graphite, and graphite is subject to ongoing oxidation and is eroded by molten aluminum. Accordingly, devices that initially perform adequately often become quickly oxidized and eroded so that their mixing and gas dispersing effectiveness diminishes rapidly; in severe cases, complete mechanical failure can occur.
The particular impeller disclosed here has proven very effective. The impeller is in the form of a rectangular prism having sharp-edged corners and multiple grooves that provides an especially effective mixing action.